This dissertation presents an alternative route to achieve ultralow thermal
conductivity in a dense solid. Thin films of disordered layered crystalline materials were
deposited using Modulated Elemental Reactants (MER) method. Cross-plane thermal
conductivity was measured using Time-Domain Thermo Reflectance (TDTR) method;
elastic properties were investigated using picosecond acoustics. The results are applied
to reducing the thermal conductivity in misfit layer materials and multilayers containing
disordered layered crystalline materials.
The cross-plane thermal conductivity of thin films of WSe2 is as small as 0.05 W
m-1 K-1 at room temperature, 30 times smaller than the c-axis thermal conductivity of
single-crystal WSe2 and a factor of 6 smaller than the predicted minimum thermal
conductivity for this material. The ultralow thermal conductivity is attributed to the
anisotropic bonding of the layered WSe2 and orientational disorder in the stacking of
well-crystallized WSe2 sheets along the direction perpendicular to the surface.
Disordering of the layered structure by ion bombardment increases the thermal
conductivity.
I measured the room-temperature, cross-plane thermal conductivities and
longitudinal speeds of sound of misfit-layer dichalcogenide films [(PbSe)m (TSe2)n]i (T =
W or Mo, m = 1-5, n = 1-5) synthesized by the MER. The thermal conductivities of
these nanoscale layered materials are 5-6 times lower than the predicted minimum
thermal conductivity Λmin of PbSe. Thermal conductivity decreases with increasing
content of the main source of anisotropy in the sample, the layered chalcogenide, and it is
largely unaffected by variations in superlattice period.
I investigated the lower limit to the lattice thermal conductivity of Bi2Te3 and
related materials using thin films synthesized by MER. The thermal conductivities of
single layer films of Bi2Te3 , Bi2Te3 and Sb-doped Bi2Te3 and multilayer films of
(Bi2Te3)m(TiTe2)n and [(BixSb1-x)2Te3]m(TiTe2)n are measured by TDTR; the thermal
conductivity data are compared to a Debye-Callaway model of heat transport by acoustic
phonons. The homogeneous nanocrystalline films have average grains sizes 30 < d < 100
nm as measured by the width of the (003) x-ray diffraction peak. Multilayer films
incorporating turbostratic TiTe2 enable studies of the effective thermal conductivity of
Bi2Te3 layers as thin as 2 nm. In the limit of small grain size or layer thickness, the
thermal conductivity of Bi2Te3 approaches the predicted minimum thermal conductivity
of 0.31 W m-1 K-1. The dependence of the thermal conductivity on grain size is in good
agreement with the Debye-Callaway model. The use of alloy (Bi,Sb)2Te3 layers further
reduces the thermal conductivity of the nanoscale layers to as low as 0.20 W m-1 K-1.